The active matrix substrate employed in a liquid crystal display device and the like is equipped with a switching element in each pixel, such as a thin film transistor (hereinafter referred to as “TFT”). Conventionally, a TFT with an amorphous silicon film as an active layer (hereinafter referred to as “amorphous silicon TFT”) or a TFT with a polycrystalline silicon film as an active layer (hereinafter referred to as “polycrystalline silicon TFT”) have been widely used as the switching element.
In recent years, there have been attempts to use materials other than amorphous silicon and polycrystalline silicon for the active layer of TFTs. For example, Patent Document 1 discloses a liquid display device that has an active layer of TFTs by using an oxide semiconductor film such as InGaZnO (oxide, which includes indium, gallium and zinc). The aforementioned TFT is called an “oxide semiconductor TFT.”
The oxide semiconductor TFTs are possible to actuate faster than the amorphous TFTs. Furthermore, because the oxide semiconductor film is formed by a simpler process than the polycrystalline silicon film, the oxide semiconductor film may also be applied to devices that need a large surface. Therefore, the use of the oxide semiconductor TFTs in display devices and the like are underway as an active element that is possible to manufacture while reducing manufacturing processes and manufacturing costs, and that operates at a higher switching performance.
Furthermore, because the oxide semiconductor has high electron mobility, the oxide semiconductor may achieve at least the equivalent performance of conventional amorphous silicon TFTs even if the size is comparatively small. Because of this, the usage of the oxide semiconductor TFTs enable a reduction in an area occupied by the TFTs within a pixel region of display devices and the like, resulting in the possibility to improve the pixel aperture ratio. Hence, there is a possibility to display a higher brightness or reduce light intensity of a backlight and achieve low power consumption.
There is difficulty increasing the aperture ratio of pixels because of a minimum width of the wiring (process rule), especially for small and high-definition liquid display devices used in smartphones and the like. Thus, improving the pixel aperture ratio using the oxide semiconductor TFTs is advantageous because a high-definition display can be achieved while reducing power consumption.